System for efficient recovery of node-b buffered data following mac layer reset

ABSTRACT

A method and system for the UE and RNC to reduce transmission latency and potentially prevent loss of PDUs upon a MAC layer reset. UE generation of the status PDU is coupled with the MAC layer reset. The RNC generates a message with a MAC reset indication. Following the MAC layer reset all PDUs stored in the UE MAC layer reordering buffers are flushed to RLC entities and then processed by RLC entities prior to the generation of a PDU status report. The PDU status report provides to the RNC the status of all successfully received PDUs. Upon reception of a PDU status report in the RNC, missing PDUs are realized and retransmitted to the UE.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/616,331 filed Jul. 9, 2003, which claims priority from U.S.Provisional Patent Application No. 60/410,737 filed Sep. 12, 2002, whichare incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to the field of wireless communications.More specifically, the present invention relates to efficient recoveryof data transmission between a pair of Layer 2 automatic repeat request(ARQ) peer entities following MAC layer reset of an intermediate nodefrom where data transmissions are distributed. One such example of thishandover scenario is a system employing hybrid ARQ (H-ARQ) and adaptivemodulation and coding (AM&C) techniques.

BACKGROUND

A third generation (3G) Universal Terrestrial Radio Access Network(UTRAN) comprises several radio network controllers (RNCs), each ofwhich is associated with one or more Node Bs, and each Node B isassociated with one or more cells.

The 3G FDD and TDD systems typically use the RNC to distribute, (i.e.,buffer and schedule), data transmissions to the UE. However, for thehigh speed channels of 3G cellular systems, data is distributed by theNode B. One of these high speed channels, for example, is the High SpeedDownlink Shared Channel (HS-DSCH). Since data is distributed by the NodeB, it is necessary to buffer data for transmission in Node B. When thedistributing entity (Node B) to which the UE is attached changes, thereis a potential loss of data buffered in the distributing entity. The RNCdoes not have an up-to-date status of the transmissions of Packet DataUnits (PDUs) since data is distributed by the intermediate point (NodeB). It is necessary for the UE to detect the data loss and requestretransmission, such as with a Status PDU, of lost PDUs from the RNC. Ifthe generation of the Status PDU is delayed, as a consequence, thelatency of data retransmission may be large and may not satisfy QoSrequirements.

This problem is aggravated in the HS-DSCH case since there are severalNode Bs associated with each RNC and there is a much higher likelihoodthat a mobile UE will require a Node B change than a change of RNC as aresult of UE cell handovers.

The HS-DSCH utilizes AMC to enable high-speed transmission of data andutilizes H-ARQ to increase the possibility of successful delivery ofdata. A serving HS-DSCH cell change is when the UE has to change thecell associated with the UTRAN access point that is performingtransmission and reception of the serving HS-DSCH radio link. Theserving HS-DSCH cell change is invoked when improved physical channelconditions and/or improved physical capacity is realized in an alternatecell.

There are two types of serving HS-DSCH cell changes. An Intra-Node Bserving HS-DSCH cell change is when the UE changes between two cellsthat are associated with the same Node B. An Inter-Node B servingHS-DSCH cell change is when the UE changes between two cells that areassociated with different Node Bs. In an Inter-Node B cell change, theNode B before the serving HS-DSCH cell change is called the source NodeB and the Node B after the serving HS-DSCH cell change is called thetarget Node B.

There are peer Radio Link Control (RLC) entities in both the RNC and theUE. The sending RLC entity signals a sequence number (SN) in the PDUheader, which is used by the receiving RLC entity to ensure that no PDUsare missed in the transmission. If there are PDUs missed during thetransmission, realized by out-of-sequence delivery of PDUs, thereceiving RLC entity sends a status report PDU to inform the sending RLCentity that certain PDUs are missing. The status report PDU describesthe status of the successful and/or unsuccessful data transmissions. Itidentifies the SNs of the PDUs that are missed or received. If a PDU ismissed, the sending RLC entity will retransmit a duplicate of the missedPDU to the receiving RLC.

It is also possible for the sending RLC entity to poll for a statusreport PDU from the receiving RLC entity. The polling function providesa mechanism for the sending RLC entity to request the status of PDUtransmissions. Although the H-ARQ operation removes some failedtransmissions and increases the probability of successful delivery ofdata, it is the RLC protocol layer that ultimately ensures successfuldelivery.

Due to dynamic changes in propagation conditions, the HS-DSCH cellchange must be performed rapidly to maintain quality of service. Duringthe serving HS-DSCH cell change, it is possible that the UE stopstransmission and reception in the source cell before all PDUs currentlystored in the source Node B are successfully transmitted. Since thesource Node B performs scheduling and buffering of the data, and sincethe data rates are very high, (for example 10 Mb/sec or higher), whenthe UE performs a serving HS-DSCH cell change (especially for anInter-Node B handover) there is a possibility that considerable amountsof data buffered in the source Node B will be lost. One reason for thisdata loss is that no mechanism exists within the UTRAN architecture fordata buffered at the source Node B to be transferred to the target NodeB. Upon a serving HS-DSCH cell change, the RNC has no information on howmuch, if any, data is lost since the RNC has no way to know what data isbuffered in the source Node B.

There are currently two ways that prior art systems handle the recoveryof data buffered at the source Node B. Following the HS-DSCH cellchange: 1) the RNC can explicitly poll for a status PDU from the UE; or2) the RNC can start transmitting in the target cell and theout-of-sequence delivery realized by the UE will generate the statusPDU.

In the first case, where the RNC explicitly polls for a status PDU, theRNC must first wait until the physical channel is established in the newcell. The status PDU request is then sent and is received and processedby the UE. The UE generates the status PDU and sends it back to the RNC,which processes the status PDU and determines which PDUs are in need ofretransmission.

In the second case, where the RNC just starts transmitting PDUs fromwhere it stopped in the source cell, the UE recognizes theout-of-sequence delivery of data and generates a status PDU back to theRNC. The RNC processes the status PDU and learns which PDUs are in needof retransmission.

In either of these two cases, if data buffered in the source Node Bneeds to be recovered, then a status PDU will be processed, but properreception of data retransmitted by the UE will be considerably delayed.This is due to delayed generation of the status PDU by the UE andreception of the status PDU in the RNC. If transmission is beingperformed in the RLC acknowledged mode, data is not passed to higherlayers until in-sequence delivery of data can be performed. Accordingly,the UE will be required to buffer the out-of-sequence data until themissing PDUs can be retransmitted. This not only results in a delay ofthe transmission, but requires the UE to have a memory capable of databuffering for continuous data reception until the missed data can besuccessfully retransmitted. Otherwise the effective data transmissionrate is reduced, thereby effecting quality of service. Since memory isvery expensive, this is an undesirable design constraint.

Another problem encountered with handover is the data that is bufferedwithin the UE. Within the MAC layer, there are typically a number ofH-ARQ processors that perform H-ARQ processing. As shown in FIG. 1,H-ARQ processing is a scheme comprising multiple parallel H-ARQprocessors on the transmitting side (P1 _(B)-P5 _(B)) and correspondingmultiple parallel H-ARQ processors on the receiving side (P1 _(UE)-P5_(UE)). Each processor pair, (for example P1 _(B) and P1 _(UE)),repeatedly and sequentially attempt transmission of a data block untilthe transmission is successful, to ensure that each block of data isreceived without an error. For each data block, the time required toachieve successful H-ARQ transmission varies. Since several data blocksare processed in parallel, it is possible that the sequence oftransmission is not maintained. Therefore, once a data block is receivedsuccessfully by the receiving H-ARQ processor, the data block isforwarded to a reordering buffer to provide in-sequence delivery to theRLC layer. The reordering buffers will reorder the data blocks based ontheir transmission sequence numbers before forwarding them to the RLClayer.

During Inter-Node B or Intra-Node B handovers, the RRC messages oftencarry a MAC layer reset indicator to the UE. Upon receiving a MAC layerreset indicator, the UE performs a sequence of functions including, butnot limited to, flushing out buffers for all configured H-ARQ processes,(i.e. flushing out the reordering buffers by disassembling all MAC-hslayer PDUs in the reordering buffers into MAC-d layer PDUs anddelivering all MAC-d layer PDUs to the MAC-d layer and then to itsassociated RLC entities). Upon Inter-Node B handovers (and some IntraNode B handovers), it is necessary to reset the MAC-hs layer in the UEsuch that all H-ARQ processes and all the reordering buffers are resetfor data reception from a new MAC-hs entity of the target Node B.

After a serving HS-DSCH cell change, the correct status of successful orunsuccessful received PDUs cannot be obtained until the procedure of theMAC layer reset is completed and the data blocks are processed by theRLC.

It would be desirable to have a system and method where data buffered inthe UE can be accounted for in order to properly maintain user qualityof service requirements.

SUMMARY

The present invention is a method and system for the UE and RNC toperform a series of actions in order to reduce transmission latency andpotentially prevent loss of PDUs upon a MAC layer reset. UE generationof the status PDU is coupled with the MAC layer reset. The RNC generatesa signaling message with a MAC reset indication. Following the MAC layerreset due to reception of a MAC layer reset request, all PDUs stored inthe UE MAC layer reordering buffers are flushed to RLC entities and thenprocessed by RLC entities prior to the generation of a PDU statusreport. The PDU status report provides the status of all successfullyreceived PDUs to the RNC. This provides fast generation of a PDU statusreport. Upon reception of a PDU status report in the RNC, missing PDUsare realized and retransmitted to the UE.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a block diagram of a prior art H-ARQ process.

FIG. 2 is a flow diagram of an efficient procedure in accordance withthe present invention for efficient recovery of UE buffered datafollowing an HS-DSCH cell change.

FIG. 3 is a flow diagram of a first alternative method whereby the RNCwaits for a status PDU prior to initiating a transmission of new data inthe target cell.

FIG. 4 is a flow diagram of a second alternative method whereby the RNCwaits for a trigger prior to initiating a transmission of new data inthe target cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred embodiments of the present invention will be describedwith reference to the drawing figures wherein like numerals representlike elements throughout.

Referring to the flow diagram of FIG. 2, a first embodiment of thepresent invention which comprises a method 10 of determining the statusof the PDU transmissions to the UE with minimal delay following a MAClayer reset condition is shown. The procedure commences with the RNCrecognizing the need to reset the UE MAC layer (step 12).

One possible cause for a UE MAC layer reset is in the event of a servingHS-DSCH cell change. The RNC informs the Node B of the HS-DSCH cellchange (step 14) in the case of an Inter-Node B serving HS-DSCH cellchange, and also in the case of an Intra-Node B serving HS-DSCH cellchange where the source Node B is the same as the target Node B, butwhere transmission queues cannot be rerouted from the source to targetcell. In both of these cases, a MAC reset is required. Along with theHS-DSCH cell change indication, the UE is informed of the MAC layerreset requirement by the RNC, as indicated via a Radio Resource Control(RRC) message (step 16). It should be noted that it is also possible toinvoke step 16 in advance of step 14 with no adverse consequences.

Those of skill in the art would realize that there are many causes for aMAC layer reset other than the HS-DSCH cell change, where the method 10for the RNC to determine PDU transmission status following MAC resetapplies. For example, a MAC layer reset may be warranted any time theNode B H-ARQ processes need to be reinitialized.

Within the RRC message, there is an identifier for the MAC layer toperform a reset. This identifier may be part of the serving HS-DSCH cellchange procedure, or may be part of any other procedure that results inresetting of the MAC layer in Node B and the UE in either an Inter-NodeB cell change or an Intra-Node B cell change. It would be understood bythose of skill in the art that there are many aspects to the MAC layer,including the MAC-hs layer and the MAC-d layer. For simplicity indescribing the present invention, reference will be made hereinaftergenerally to the MAC layer.

The HS-DSCH is a data transport channel. For each data transportchannel, there can be a plurality of RLC instances. The RLC instancesare essentially logical channels which may be mapped to the sametransport channel; for example, several RLC entities may be mapped to asingle transport channel HS-DSCH. An RLC instance is called AcknowledgedMode (AM) if ARQ is used to ensure correct transmission between the peerRLC instances. A pair of AM RLC entities uses status PDUs for thereceiver to indicate to the sender the status of successfultransmissions of PDUs. Following the occurrence of the HS-DSCH cellchange and a MAC layer reset, each of the AM RLC instances associatedwith a particular HS-DSCH generate a status PDU.

The RRC message along with the MAC layer reset indicator is received andprocessed by the RRC in the UE (step 18). The UE RRC checks whether aMAC layer reset indicator is set and, if so, the RRC informs the MAClayer of the MAC layer reset request (step 20). Upon reception of theMAC layer reset request, the MAC layer resets and in addition to othertasks, flushes all PDUs stored in its reordering buffers to the RLCentities mapped to the HS-DSCH (step 22). All flushed PDUs are thenprocessed by the RLC instances mapped to HS-DSCH (step 24) beforegeneration of a PDU status report (step 26).

RLC processing of PDUs stalled in reordering buffers before thegeneration of a PDU status report is necessary to provide accurate andcomplete transmission status to the RNC. If PDU status reports aregenerated early, (i.e. before all PDUs buffered in MAC reordering queuesare processed by the RLC instances), some PDUs may be incorrectlyindicated as not being received, and as a result unnecessary PDUretransmissions may be generated by the RNC.

There are several ways to ensure that all PDUs have been processed bythe RLC so that the AM RLC entities will be able to obtain the correctstatus of all successfully received PDUs. First, the MAC layer forwardsPDUs in-sequence from each reordering queue and then generates an“end-of-PDU” indication for each reordering queue.

In a second alternative, the last PDU from each reordering queue has aspecial indicator. These are reports of the status of the RLC PDUsreceived in the UE.

In a third alternative, the RLC confirms to the MAC layer when PDUs havebeen processed, and following the processing of all PDUs, the MAC layergenerates a PDU status request to the RLC. It should be understood thatthere are numerous ways to coordinate processing between the MAC layerand the RLC to ensure all PDUs are processed by the RLC beforegeneration of the PDU status message.

After receiving and processing the PDUs, the AM RLC generates a PDUstatus report (step 26) which indicates all successfully orunsuccessfully received PDUs. The PDU status report is generated foreach AM RLC instance mapped to the HS-DSCH. A PDU status report may begenerated even though no PDUs were forwarded from the MAC layer for thatAM RLC instance. The UE then autonomously sends the PDU status reportfor each AM RLC instance associated with the HS-DSCH to the RNC.

In the RNC, assuming that the AM RLC and MAC entities are not informedto stop transmitting PDUs due to the MAC layer reset, the RNC continuesto transmit PDUs regardless of the MAC layer reset. Upon reception ofthe PDU status report for each AM RLC instance associated with theHS-DSCH, the RLC instances in the RNC process the status reports (step28) to determine lost PDUs and generate PDU retransmissions as necessaryto ensure successful delivery (step 30). To achieve quality of servicerequirements, the retransmissions may take precedence over currenttransmission processing.

It should be understood that the need for the MAC layer reset is commonwith the need to generate a PDU status report. Indication of eitherrequirement, or some common indication, can be signaled to the UE toinvoke both the MAC layer reset and generation of the PDU status report.The UE will then perform each function in the sequence described.

This first embodiment of the present invention as shown in FIG. 2permits the RNC to keep transmitting to the UE while the data path isswitched from one radio link to another. However, in accordance with twoalternative embodiments of the present invention, shown in FIGS. 3 and4, data transmissions are halted upon an HS-DSCH cell change or otherevents that result in the need for MAC layer reset until the occurrenceof a subsequent event. It should be noted that the steps shown in FIGS.3 and 4 which have the same element numbers as the steps shown in FIG. 2are identical. Accordingly, the description of those steps will not berepeated when referring to FIGS. 3 and 4.

A second embodiment of the present invention comprises a method 40 fordetermining the status of PDU transmissions to the UE with minimal delayfollowing a MAC layer reset condition and is shown in FIG. 3. After theRNC recognizes the need for a MAC layer reset (step 12) and the Node Band UE are notified (steps 14 and 16), the RNC halts all downlinkHS-DSCH transmissions (step 17). Note that step 17 may occur in advanceof step 14 or 16 without any adverse consequences. The RNC subsequentlyreceives the PDU status report (step 32). The PDU status reportindicates the PDUs lost as a result of the MAC reset and potentiallyadditional PDUs lost in the source Node B the case of an HS-DSCH cellchange. The PDU status report is then processed (step 34) and themissing PDUs are retransmitted to the UE (step 36). The RNC initiatestransmission in the new cell by scheduling transmission of lost PDUsthat require retransmission first. The RNC then resumes PDUtransmissions (step 38) at the point where transmissions were previouslystopped at step 17. Note it is also possible that steps 36 and 38 areperformed simultaneously.

Referring to FIG. 4, a third embodiment of a method 50 in accordancewith the present invention is shown. This method 50 is similar to themethod 40 shown in FIG. 3. However, instead of restarting the downlinkHS-DSCH transmissions to the UE in response to the receipt of a PDUstatus report at step 32 as shown in FIG. 3, the method 50 of thisembodiment of the present invention restarts transmissions upon thereceipt of a “trigger”, or a pre-determined event (step 19). In a firstexample, the trigger may comprise the establishment of the transportchannel in UTRAN which, as would be understood by those of skill in theart, is accomplished by an RNC with the new “target” Node B signalingprocedure. The reception in the RNC of a confirmation generated by theNode B is used as the trigger.

In a second example, the trigger may comprise reception or detection ofthe “in-sync” indication. Upon establishment of dedicated resources inthe target Node B, an “in-sync” indication may be determined in the NodeB when the assigned physical channels are determined to be available fortransmission in the Node B. Indication of this event is relayed to theRNC and can then be used as a trigger

In a third example, the trigger may comprise completion of the RRCprocedure, (i.e., confirmation of the RNC reception of the UE RRCmessage). The RRC message signaled in step 16 results in an RRCconfirmation message that is generated by the UE and sent to the RNC.When this message is received at the RNC it can be used as a trigger.

It should be noted that there are many different signals that are sentbetween the UE and the RNC, and any of these may be selected as desiredby the user to act as the trigger in accordance with the presentinvention. Accordingly, the aforementioned three examples are intendedto be instructive rather than restrictive. Regardless of the form of thetrigger, after the trigger is received the RNC restarts HS-DSCHtransmissions (step 21).

1. A method for use a radio network controller (RNC), the methodcomprising: generating a medium access control (MAC) layer resetindication; transmitting a radio resource control (RRC) message to auser equipment (UE), the RRC message having an identifier indicatingthat a medium access control high speed (MAC-hs) is to be reset; andreceiving one or more radio link control (RLC) status report from theUE, wherein the RLC status report indicates the status of RLC protocoldata units (PDUs) received by the UE.
 2. The method of claim 1 whereinthe RRC message is associated with a high speed downlink shared channel(HS-DSCH) inter-Node B cell change.
 3. The method of claim 1 whereintransmission of PDUs from the RNC to the UE is halted until a triggeringevent occurs.
 4. The method of claim 3 wherein the triggering event is areceipt of the RLC status report.
 5. The method of claim 3 wherein thetriggering event is an establishment of new channel for the UE.
 6. Themethod of claim 3 wherein the triggering event is an indication ofsynchronization of a new Node B with the UE.
 7. The method of claim 3wherein the triggering event is the completion of a radio resourcecontrol procedure.
 8. The method of claim 1 further comprising the RNCretransmitting missing PDUs to the UE based on the status report.
 9. Aradio network controller (RNC), the RNC comprising: a controllerconfigured to generate a medium access control (MAC) layer resetindication; a transmitter configured to transmit a radio resourcecontrol (RRC) message to a UE, the RRC message having an identifierindicating that a medium access control high speed (MAC-hs) is to bereset; and a receiver configured to receive one or more radio linkcontrol (RLC) status report from the UE, wherein the RLC status reportindicates the status of RLC protocol data units (PDUs) received by theUE.
 10. The RNC of claim 9 wherein the RRC message is associated with ahigh speed downlink shared channel (HS-DSCH) inter-Node B cell change.11. The RNC of claim 9 wherein a transmission of PDUs from the RNC tothe UE is halted until a triggering event occurs.
 12. The RNC of claim11 wherein the triggering event is a receipt of the RLC status report.13. The RNC of claim 11 wherein the triggering event is an establishmentof new channel for the UE.
 14. The RNC of claim 11 wherein thetriggering event is a completion of radio resource control procedure.15. The RNC of claim 9 wherein the transmitter of the RNC is furtherconfigured to retransmit missing PDUs to the UE based on the statusreport.